Abstract
Capitalizing on some earlier work, this paper presents a novel Monte Carlo-based approach that allows estimating the neutron noise induced by stationary perturbations of macroscopic cross-sections in the frequency domain. This method relies on the prior computation using Monte Carlo of modified Green’s functions associated to the real part of the dynamic macroscopic cross-sections, mimicking equivalent subcritical problems driven by external neutron sources. Once such modified Green’s functions are estimated, the neutron noise induced by any type of perturbations can be recovered, by solving a linear algebra problem accounting for the interdependence between the real and imaginary parts of the governing balance equations. The newly derived method was demonstrated on a large homogeneous test system and on a small heterogeneous test system to provide results comparable to a diffusion-based solver specifically developed for neutron noise applications. The new method requires the specification by the user of the real part of the Fourier transform of the macroscopic cross-sections. This is accomplished using ACE-formatted cross-section files defined by the user. Beyond this input data preparation, no change to the Monte Carlo source code is necessary. This represents the main advantage of the proposed method as compared to similar efforts requiring extensive modifications to the Monte Carlo source code.
Highlights
Nuclear reactor core monitoring using non-intrusive techniques is becoming increasingly important, due to the ageing fleet of reactors and, correspondingly, operational problems being more frequently encountered
Most of the diagnostic tasks are based on unfolding from the detector readings the nature and possibly location of the driving perturbation or anomaly. This unfolding requires the prior determination of the reactor transfer function, i.e. the function giving the response of the system to localized perturbations
A method allowing the estimation, using Monte Carlo techniques, of the neutron noise induced by stationary fluctuations of the macroscopic cross-sections was developed
Summary
Nuclear reactor core monitoring using non-intrusive techniques is becoming increasingly important, due to the ageing fleet of reactors and, correspondingly, operational problems being more frequently encountered. Some earlier work performed by one of the present authors demonstrated the possibility to perform Monte Carlo-based noise calculations without modifying the source code [5]. The main advantage of the new framework lies with the fact that those modified Green’s functions only involve real quantities, are equivalent to balance equations obtained for static subcritical systems driven by external neutron sources and can be, again, readily obtained from Monte Carlo without source code modifications. In both cases, the Monte Carlo solutions are compared to the ones obtained using the CORE SIM tool [6], which is a diffusion-based deterministic solver developed and verified for noise calculations [7]. Some conclusions and recommendations for future work are presented
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